Q. If you use most of a previous paper (of your own) into a new one (new class), is this considered plagiarism?

Plagiarizing yourself by copying and pasting large chunks of previously written material without citing it may be considered academically dishonest by most professors. The reason is, professors give you specific assignments with the hopes that you will do fresh work each time.

To ensure you are acting with academic honesty, you should ask your professor if you are allowed to quote or paraphrase your ideas from your old paper as long as you cite it. This will follow the same procedures as citing from any source, which means that the core of the new paper must differ from the old, and you can't use too large of portions of the old paper.